By default, the single Docs will show a Table of Contents on the sidebar. If you want to add other widgets to the sidebar on the single Docs, you can.
In your WordPress dashboard, at Appearance –> Widgets, you’ll see the Docs Widget Area. You can add any widgets to the Docs Widget Area, but then you’ll have to manually add the Table of Contents. To do this, add the widget named “Organized Docs Table of Contents” which is the Table of Contents widget.